What year did World War II officially start?

World War II officially started in 1939 when Germany invaded Poland on September 1st. This invasion prompted Britain and France to declare war on Germany, marking the beginning of a global conflict that would involve many countries worldwide. The significance of this date lies in its role as a catalyst for widespread military engagements and geopolitical changes.

The other years mentioned relate to significant events during the war: 1941 marks the entry of the United States into the conflict following the attack on Pearl Harbor, 1945 is noted for the end of the war with the surrender of Germany and Japan, and 1937 is often associated with earlier conflicts in Asia that eventually contributed to the broader war, but none of these years represent the official beginning of World War II as recognized by historians.